In the past, opto-electronics engineers have utilized optical fibers in numerous sensor systems, such as gyroscopes, microphones, hydrophones, vibration sensors, and tachometers, etc.
Typically, these sensor systems would include an optical sensor coupled to a light source by an optical fiber and would also typically include a fiber-optic coupler between the source and sensor for providing optical signals to a detector.
While this approach is common and has been utilized successfully, it does have several shortcomings. First of all, the use of fiber-optic couplers or beam splitters result in a significant loss in optical signal strength. This often requires a higher gain in the photodetector preamplifier, which tends to limit the readout band width. Additionally, the use of a fiber-optic coupler adds additional expense, especially when expensive polarization maintaining fiber optic couplers are used.
In Arditty et al U.S. Pat. No. 4,842,409, teaches that the losses associated with a coupler or beam splitter between the light source and sensor may be eliminated by using the light source as both a generator and a light amplifier. The light source is described as a laser or superluminescent diode with its front face optically coupled to the mode filtering input fiber to a fiber-gyro, and its rear face coupled to a photodetector such as a PIN diode. In optical sensors such as the fiber-optic gyro output signal fed back into the sensor will result in significant distortion of the output signal. There are several locations where back-reflections and amplified back-reflections can cause feedback into the sensor causing these distortion errors. Because solid state light sources such as the InAlGaAs SLD currently in use have optical indexes significantly higher than the fiber typically used in sensors, back-reflection can occur from the interface of the fiber to the diode facet. If an air gap exists between the diode and the fiber, the reflection can be even higher. A second, potentially larger, source of reflected signal can result from the back facet of the diode. Because the signal returning from the sensor is amplified as it passes through the light source, any slight reflections from the back facet will be returned to the sensor multiplied by twice the light source single pass gain. Reflections from the photodetector located next to the rear facet will result in a similar amplified return signal.
Consequently, there exists a need for improvement in fiber-optic sensor systems which reduce the need for higher gains in the detector preamplifier and eliminates the fiber-optic coupler and its associated expense and reduces reflections back into the light source.